The present invention is directed to semiconductor apparatus fabrication, and especially to effecting finishing operations with workpieces, such as semiconductor wafers.
A process often used for carrying out finishing operations with semiconductor workpieces such as semiconductor wafers is a chemical mechanical polishing (CMP) process. The process is sometimes referred to as chemical mechanical planarization and is also referred to in that context as CMP. For purposes of this application chemical mechanical polishing and chemical mechanical planarization are regarded as interchangeable terms describing substantially the same process.
During CMP processing certain film characteristics of surfaces being finished are known to cause excessive polishing with little down-force or slow platen speeds. Down-force and pressing force are terms used to describe the force with which a polishing or finishing tool is urged against a workpiece during finishing operations. Platen speed refers to speed of a tool platen holding a finishing tool, or refers to speed of a workpiece platen holding a workpiece, or refers to a resultant speed experienced between a finishing tool and a workpiece when both a tool platen and a workpiece platen are moved during finishing operations.
Films which exhibit a relatively high amount of polishing with little down-force or slow platen speeds are typically low dielectric constant (low-k) films. When such low-k films are subjected to polishing upon a semiconductor wafer they are prone to having some areas on the wafer polish much faster than other areas on the wafer. Certain slurry materials introduced between a finishing tool and a workpiece may also contribute to fast polishing areas, or hot spots. Fast-polishing areas, or hot spots, typically occur at or near the center of the workpiece but not exclusively so. Such hot spots often cause damage to features underlying the surface being finished.
Attempts have been made to provide for cooling of platens (workpiece platens, or tool platens or both platens) to avoid hot spots and thereby avoid uneven results of finishing operations. It has proven difficult to maintain a constant workpiece surface temperature through the thickness of the workpiece when the workpiece platen is cooled. When cooling the tool platen it proved difficult to maintain a constant workpiece surface temperature through the thickness of the tool (for example, the tool is commonly configured as a polishing pad).
A contrary condition working against the desire to cool the workpiece surface in order to avoid hot spots is a requirement for an elevated operating temperature range sufficient to promote acceptably rapid polishing rates. It has proven difficult to cool one or both platens sufficiently to avoid hot spots and still maintain the elevated temperatures that are required to sustain a reasonable polish rate.
There is a need for a method for effecting a finishing operation on a semiconductor workpiece that reduces localized heating of the workpiece.
The inventors have discovered that carrying out a finishing operation for a semiconductor workpiece, such as a semiconductor wafer, in a manner to provide for breaking up the polishing time by the finishing tool against the workpiece into higher down-force periods and lower down-force periods yields a more evenly finished end product than is provided using prior art finishing techniques in which a substantially constant down-force is exerted against the workpiece for the duration of the finishing process step. Using the novel method for finishing semiconductor workpieces disclosed herein, a manufacturer experiences fewer manifestations of product damage from hot spots than was attainable using prior art, constant down-force finishing techniques.
The variance of down-force by the finishing tool against the workpiece may be effected by several ways including, but not limited to lifting the finishing tool clear of the workpiece, varying down-force, or otherwise cycling or modulating down-force exerted by the finishing tool against the workpiece. Such modulation or interruption of down-force by the finishing tool against the workpiece permits, or at least encourages, cooling of the workpiece, cooling of the finishing tool, redistribution and cooling of slurry material between the finishing tool and the workpiece and other stabilizing phenomena between the finishing tool and the workpiece that promote even finishing of the workpiece during finishing operations. Planned transfer of a workpiece from one platen to another platen during finishing operations may be employed as a way to effect the desired interruption of down-force by the finishing tool against the workpiece. Such an occasion of changing workpiece platens also provides an opportunity to introduce other changes or interruptions to the finishing process to enhance the effects of varying down-force, such as introducing another slurry material between the finishing tool and the workpiece or establishing a different platen speed.
Depending upon a variety of factors (e.g., materials on the workpiece surface, slurry material employed, thickness of materials involved, or other factors), if the chosen adjustment to down-force is interruption of down-force as by lifting the finishing tool clear of the workpiece, the duration of an interruption of down-force may range from about 0.1 second to about 120 seconds. Time intervals of down-force interruptions need not be equal over the duration of a finishing operation. Also depending upon various and several factors, the number of interruptions during a finishing operation may range from one time to approximately 100 times.
A method for effecting a finishing operation on a semiconductor workpiece situated in a finishing apparatus that includes a finishing tool configured for pressingly engaging the workpiece with a pressing force for abradingly removing material from the workpiece includes the steps of: (a) situating the finishing tool to operate against the workpiece; (b) operating the finishing tool with a pressing force to effect the abrading removal; (c) measuring at least one parameter associated with the finishing operation to determine at least one parametric value for the at least one parameter; (d) modulating the pressing force according to a predetermined relationship between the pressing force and the at least one parametric value; and (e) repeating steps (c) and (d) until the finishing operation is complete.
The method of the present invention permits employment of aggressive polishing parameters (e.g., high platen speed, high down-force and abrasive slurry materials) that are required for high speed finishing without producing the hot spots caused by high temperatures ordinarily created by applying such aggressive finishing parameters. The method also encourages even distribution and cooling of slurry material between a tool and a workpiece in finishing operations.
Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention.